In order to satisfy the cell phone customer's desire for personal customization, the handset market offers a variety of different means of customization. Some handset manufactures sell phones in several colors. Such marketing requires the cell phone user to select the color of the phone during the sell process with no flexibility to change during the contract period. Other manufacturers offer replacement covers to the most popular types of phones. This solution meets the need for customization on one hand and the need to adopt and change color during the ownership period. Yet another customization option is the use of covers. Covers are made to fit popular phone modules. They usually are of silicon or rubber base and their main purpose is to protect the body of the phone and allow certain color collections. The user can replace the cover as many times as he may want, with no need for professional assistance, with high flexibility and re-use. A problem with this technique is the relatively high cost and small color selection of covers.
An electronic skin or cover on a cell phone housing or case to allow the cell phone customer to instantly change its color is a feature desired by nearly every handset manufacturer. Referred to as an electronic skin, such a device allows the consumer instant freedom to change the color and personalize their phone, laptop, iPod® music player or other electronic device. An electronic skin can indicate functional features with color such as ring tones (silent alert), call waiting and missed calls on cell phones. Customers of hand held devices are attracted to colorful designs that can reflect personal style and select them for some of the same reasons that they select clothing colors and fashion accessories. Patterns, in addition to color, give the consumer added capability of adding logos or even photos. The primary motivation for the incorporation of electronic skins is product differentiation, which is important in mobile consumer electronics for increasing the manufacturer's market share and margin.
The concept of electronically changing the color of a handheld or portable electronic device is not new and cell phone manufacturers have considered all available display technologies for this application. The important requirement of low power eliminates all of the emissive display technologies such as OLED, electroluminescent displays and backlit LCDs. Emissive displays such as these are not suitable technologies for electronic skins on cell phones since they draw substantial power from the cell phone battery and quickly drain it leaving little power available for the functional use of the cell phone or player. Electronic skins used on portable electronics therefore should employ a color reflective technology in order to avoid the power loss involved in generating light necessary for emissive display technologies.
There are many reflective display technologies. However, many of them are not attractive for an electronic skin application because of many requirements: high reflectivity, wide color gamut, low cost, mechanical robustness, and thin conformable geometries in addition to low-power. An old and well known bistable technology is the electrochromic display technology. This technology, however, has insufficient color gamut and, even though bistable, is not a low power option in that it is current driven requiring significant power from the battery to change the color. The bistable electrophoretic technology is used in black and white displays and covers but it is not yet developed for color. Further, it is not attractive for conformable color electronic skin development as it would require high resolution patterning of the primary colors which adds considerably to the cost and limits the brightness to less than 33%. Electromechanical displays also suffer from this 33% reflectance limitation. Electrowetting lenses (U.S. Pat. No. 6,934,792) have been proposed; however, this scheme results in low reflectivity, the inability to mix colors, and suffers from objectionable layer thickness. Polarizer-based LCD technologies (e.g., Zenthal Displays, bistable STN) have been made bistable for reduced power but these use color filters to achieve color and as such can suffer from not being reflective enough for electronic skin applications.
A potential reflective technology that has been considered for electronic skins is the electrowetting or electrofluidic display technology. One major limitation of this technology for electronic skins is that power must be applied repetitively to display a particular color. While the power drain is small compared to an emissive display, it nonetheless becomes quite significant and excessive on an area the size of an electronic skin since it must be applied continually or continually refreshed in order to maintain the selected color. The color of the phone must be maintained even while the phone is not in operation, a serious problem for this technology. Another serious problem is the complex cell structure further adds excessive cost in an electronic skin application.
At this time the bistable cholesteric liquid crystal technology is the preferred technology and the one most sufficiently advanced for electronic skin applications. It is bistable (see U.S. Pat. Nos. 5,347,811 and 5,453,863) so that no power is required to maintain a selected color on the electronic skin. One of the stable states is a reflective state in which the material takes on a planar texture. In that texture the material reflects a preselected color determined by the components that make up the material. Another stable state is one in which the material takes on the so called focal conic texture that is substantially transparent in the electronic skin application. One can electronically switch between the two states with a voltage pulse of different magnitude. A third state is possible with a continuously applied voltage, called the homeotropic texture that is more transparent than the focal conic texture. Because the planar texture only reflects a selected color and is transparent to other colors the technology is amenable to vertical stacking the layers to achieve a film that can be switched to a multiplicity of colors, U.S. Pat. No. 6,377,321. The intensity of the reflected colored light can be adjusted by the voltage pulse so that a vertical stack of the three primary colors, red, green and blue can be mixed to show any color; 4096 colors have been demonstrated. Unlike the electrofluidic technology, there is no cell structure and an electronic skin is simply made at low cost by coating the display material directly on a flat substrate. The material for the electronic skin application is made by dispersing the cholesteric liquid crystalline material in a polymeric matrix making it rugged for skin use and aiding its manufacturability on a web line. Light traveling through the cholesteric liquid crystal layer of cholesteric liquid crystal displays, is absorbed by any of the following items located at the back of the display: a back color layer (U.S. Pat. No. 5,493,430 incorporated herein by reference); black paint coated on an inside of a back glass or polymer substrate; and a nonreflective back substrate such as fabric, a solar cell or an active matrix. Another important advantage of the cholesteric technology over other reflective technologies, such as the electrophoretic or electrofluidic technologies, is that bistable cholesteric electronic skins can show high resolution patterns such as logos or pictures simply by patterning the electrodes and not requiring an expensive active matrix backplane. Cholesteric electronic skins can be electronically multiplexed to provide patterned full color images without the active matrix, which is suitable for low cost cell phone skins.
Another application of cholesteric displays is writing tablets. A variety of writing tablet technologies have been proposed over the years. In one technology described in U.S. Pat. No. 6,104,448 an image can be written on a tablet with an untethered stylus. The pressure of the stylus changes a transparent texture of the cholesteric liquid crystal to the color reflective texture at the stylus tip. When viewed on the black background (e.g., a black coating on a back substrate), a line traced by the stylus in the color reflective texture is bright and contrasts strongly against the transparent texture which appears black on the black background. The bistability of these two textures allows the image to remain on the tablet until erased. The image is instantly erased by application of a voltage to two transparent electrodes that sandwich the cholesteric material in between. An applied voltage drives the color reflective texture created by the stylus tip back to the transparent texture thereby erasing the image. A new image can then be traced by the stylus.
Up to now, electronic skins have only been considered for the housing of the electronic device; see for example published U.S. patent applications Pub. Nos. US 2003/0160741 and US 2008/0074383. In recent years, however, the electronic display on the device is becoming larger, and by adding touch screen capability, the display can replace buttons and keys on the device. The display therefore now occupies nearly all of the area on one side of the device leaving little remaining housing for the electronic skin to cover. When the device is not in use, the display is black as is the entire front surface of the device. The device of this disclosure addresses that issue by being an electronic skin that can overlay or cover the display. The inventive skin is substantially transparent when the display is turned on and in use; however, can be a decorative color when the display is off and fully functional as an electronic skin or writing tablet skin.